This invention relates to a solid electrolyte capacitor of the type having a dielectric oxide film formed on the surface of an anode body of a valve metal, a metal oxide electrolyte layer covering the dielectric oxide film and a cathode collector layer covering the electrolyte layer, and more particularly to the material and construction of the cathode collector layer.
A solid electrolyte capacitor of the above described type is quite popular. Usually tantalum or aluminum is used as the anode material, and the dielectric oxide film is formed by anodization of an anode body which may take the form of a sintered mass, plate, rod, foil or a thin film deposited on a substrate. Manganese dioxide is predominant as a semiconductive metal oxide serving as the solid electrolyte but lead dioxide too is of use. As the most familiar construction of the cathode side of this capacitor, a layer of colloidal carbon or graphite is formed on the solid electrolyte layer to serve as a cathode collector usually by the use of an aqueous dispersion of colloidal graphite, and the carbon layer is covered with a conductive layer of a two-layered structure produced by first applying a silver paint and, after drying of the paint, performing a solder coating.
Considering the role of the carbon layer as a charge collector in this capacitor, it is desired that the contact of the carbon layer with the solid electrolyte layer be as intimate as possible since, as can be predicted, the intimateness of the contact greatly influences important characteristics, particularly tan .delta. of the capacitor. When the contact is not realized with a sufficiently low contact resistance, there occurs a considerable increase in tan .delta., a loss relating to series resistance in the capacitor.
However, it is a regrettable fact that the conventional cathode collector layer given by the application of colloidal carbon is far from ideal either in the intimateness of its contact with the solid electrolyte layer or the resistance of itself. Since the carbon layer is made up of carbon particles which are roughly spherical and have usually a mean particle size of about 1 .mu.m, the contact between the carbon layer and the electrolyte layer is established actually by a nearly point-to-point contact of the individual carbon particles with the surface of the electrolyte layer, resulting in that a considerable area of the electrolyte layer surface is left uncovered if viewed microscopically. The carbon particles contact with each other, but each contact area is also a very small one. The insufficiency of covering of the electrolyte layer surface with carbon particles and the smallness of actual contact area are augmented by another fact that a manganese dioxide layer, which is most widely used as the solid electrolyte layer, has usually a porous structure with a microscopically uneven or rugged surface.
Researches have been continued after a method of forming a manganese dioxide layer with an excellent smoothness of the surface to realize a better contact of the carbon layer with the solid electrolyte layer but have not yet been fruitful from the industrial viewpoint. Even when success is attained, an improvement by this method on the electrical contact will have a limit. As an alternative way, it has been proposed to polish, for example by sand blasting, the surface of a porous manganese dioxide layer formed by a conventional pyrolysis method in order to obtain a smooth surface. However, this method too cannot solve the problem completely and, besides, has the disadvantage that a mechanical stress cast on the capacitor element during the polishing process tends to cause deterioration of the dielectric oxide film and as the result an increased leakage current and/or lowered breakdown voltage of the capacitor.
Colloidal carbon has been preferred as the material of the cathode collector layer because of being considered to fairly well satisfy the requirements to be able to permeate into and firmly adhere to a porous structure such as a manganese dioxide layer given by thermal decomposition of a manganese nitrate solution, to retain the form of fine particles after processed to give a cathode collector layer, to be good at electric conductivity and to be inexpensive. Besides, a cathode collector layer of colloidal carbon can be produced without the need of such intense heating as will cause deterioration of the solid electrolyte layer.
Some metals such as silver, copper and gold are available in a colloidal form with good permeation ability. These colloidal metals are superior to colloidal carbon in electric conductivity but as a disadvantage they are relatively large in mean particle side, i.e. several microns, compared with about one micron of colloidal carbon. Besides, these colloidal metals are far expensive than colloidal carbon.